The present invention relates to a color image forming apparatus using the electrophotographic technology such as a printer, a copy machine, a facsimile machine, or the like. More particularly, the invention relates to a color image forming apparatus in which a plurality of toner images respectively formed by a plurality of single-color toner image formers are sequentially transferred on a transferring member or a recording medium held by the transferring member.
In general, an image forming apparatus and in particular, a tandem image forming apparatus using electrophotography has a photoconductor as an image supporting member having a photosensitive layer on the outer peripheral surface thereof, a charger for uniformly charging the outer peripheral surface of the photoconductor, an exposer for selectively exposing the charged outer peripheral surface to light for forming an electrostatic latent image thereon, and a developer for giving toner to the electrostatic latent image for providing a visible image (toner image).
FIG. 7 shows an example of the tandem-type color image forming apparatus disclosed in Japanese Patent Publication No. 11-231754A. In this example, a plurality of (four) toner image formers 10a-10d each including a image supporting member 2a-2d are brought into contact with an intermediate transfer belt 1 serving as a transferring member, for sequentially transferring multiple-color toner images (for example, yellow, cyan, magenta and black) supported by the respective image supporting members 2a-2d onto the intermediate transfer belt 1. A full-color image thus formed on the intermediate transfer belt 1 is then secondarily transferred onto a recording medium P such as paper by a transfer roller 6.
There is another type of the tandem-type color image forming apparatus in which a recording medium such as paper is held and transported by a belt member serving as a transferring member so that toner image formed on the above image supporting members are sequentially transferred onto the recording medium.
In FIG. 7, the intermediate transfer belt 1 is circulated by a drive roller 3 and a driven roller 5. Therefore, the image supporting members 2a-2d are brought into contact with a slack side of the transfer belt 1.
In such a configuration, a tension roller 4 for giving a relatively large tension to the intermediate transfer belt 1 by a relatively large force F has been provided to stabilize the contact between the transfer belt 1 and the image supporting members 2a-2d. Thus, the stretching structure of the transfer belt becomes complicated so that it is difficult to reduce the size of the image forming apparatus.
Since a relatively large tension is given to the transfer belt 1 when the image forming apparatus is not activated (the transfer belt is not circulated) for a long time period, creep deformation would occur in the transfer belt, adversely affecting the later image formation.
By the way, to provide a good transfer condition of toner images (therefore providing a fine image) in the image formation apparatus as described above, it is desirable that the circumferential velocity of the image supporting member and that of the transferring member should be completely matched with each other. More practically, however, manufacturing errors and varying tolerances are usually introduced into the image supporting member, the transferring member, or the parts making up their driver units at the manufacturing stage. Thus, it is practically impossible to completely match the circumferential velocity of the image supporting member and that of the transferring member.
If the difference between the circumferential velocity of the image supporting member and that of the transferring member varies, for example, if the circumferential velocity of the image supporting member is higher than that of the transferring member at one point in time and the former becomes lower than the latter at another point in time, the transfer condition becomes remarkably unstable and a fine image cannot be obtained. Particularly, to superpose multiple-color toner images on each other for forming a full-color image as described above, a color-to-color shift occurs and the image quality is remarkably degraded.
To solve this problem, as shown in FIG. 8, Japanese Patent Publication No. 4-324881A discloses a color image forming apparatus wherein the velocity of each image supporting member is always higher than that of transferring member.
Four photoconductive drums 11 (Y, M, C, and K), each associated with a single color are brought into contact with an intermediate transfer drum 12. A yellow toner image provided by the photoconductive drum 11Y, a magenta toner image provided by the photoconductive drum 11M, a cyan toner image provided by the photoconductive drum 11C, and a black toner image provided by the photoconductive drum 11K are sequentially transferred to the intermediate transfer drum 12 to form a full-color toner image thereon, which is then transferred from the intermediate transfer drum 12 to a recording medium P.
The intermediate transfer drum 12 is driven by a driving system 14 and the photoconductive drums 11 (Y, M, C, and K) are driven by driving systems 13 (Y, M, C, and K). The driving systems 13 (Y, M, C, and K) comprise velocity converter 15 (Y, M, C, and K), respectively.
In this apparatus, to set the circumferential velocity V (Y, M, C and K) of each photoconductive drum 1 (Y, M, C and K) higher than the circumferential velocity V1 of the intermediate transfer drum 2, separate drive sources are provided. Accordingly, the driving mechanism becomes very complicated and the apparatus size is also increased.
Further, this publication is silent about fluctuation in the difference between the circumferential velocity of the image supporting members and that of the transferring member. Therefore, the transfer condition would become remarkably unstable as described above, so that a fine image cannot be provided. Particularly, when multiple-color toner images are superposed on each other for forming a full-color image as described above, a color-to-color shift would occur and the image quality is remarkably degraded.
It is therefore an object of the invention to provide a color image forming apparatus that can solve the problems as described above, can be miniaturized, and does not promote creep in a transfer belt.
It is another object of the invention to provide a color image formation apparatus which stabilizes the transfer condition from an image supporting member to a transferring member with a simple driving system.
In order to achieve the above objects, according to one embodiment of the present invention, there is provided a color image forming apparatus, comprising:
a drive roller and a driven roller;
a looped belt member stretched and circulated by at least the drive roller and the driven roller so as to have a slack side and a tensed side; and
a plurality of image supporting members, each supporting a single color toner image thereon, and abutting onto the slack side of the belt member to define a transferring position at which the toner image is transferred onto either the belt member or a recording medium held by the belt member, while being rotated, wherein:
a circumferential velocity of each image supporting member is determined so as to be higher than a circulation velocity of the belt member.
In this configuration, slack in the transfer belt originally occurred in the winding release point of the drive roller, is taken up so that the transfer belt is placed in a tensed state between the respective transferring positions. Accordingly, the running condition of the transfer belt becomes stable without wrinkles or slack, and the transfer position is fixed uniquely to a predetermined position.
Further, because of this configuration, an additional tension roller as employed in the related art is not needed so that the stretching structure of the transfer belt can be simplified (the transfer belt may be looped only on the drive roller and the driven roller) and correspondingly, the size of an image forming apparatus can be reduced. Moreover, since a considerably large tension due to the provision of the tension roller does not act on the transfer belt, creep deformation is reduced or eliminated in the transfer belt even if the image forming apparatus is not activated for a long time period. Consequently, image formation and quality can be improved.
Here, it is preferable that the rotation velocities of the image supporting members are substantially the same as each other.
In another embodiment, the circumferential velocity of an image supporting member which is further from the driving roller is higher than a circumferential velocity of an image supporting member which is closer to the driving roller.
This additionally allows the transfer belt to be maintained in a tensed state between the adjacent image supporting members, allowing the running condition of the transfer belt to become reliably stable between the adjacent image supporting members.
In a preferred embodiment, the apparatus further comprises:
a first gear train, which rotates the drive roller to circulate the belt member;
a second gear train, which rotates the image supporting members, while being connected to the first gear train with no branch; and
a single drive source, which drives the first gear train to thereby drive the second gear train.
In this configuration, since both of the first gear train and the second gear train are driven by the single drive source, the mechanical structure is remarkably simplified which allows for the apparatus size to be reduced. Furthermore, backlash in the first gear train does not occur although the circumferential velocity of the image supporting member is higher than the circulating velocity of the transfer belt.
Also according to the invention, a surface hardness of the belt member may be greater than a surface hardness of the respective image supporting members. Alternatively, or in addition to this surface hardness relationship, a surface roughness of the belt member may be greater than a surface roughness of the respective image supporting members.
In addition, an abrasive may be applied on a surface of the belt member.
In any of the above configurations, whenever the surface of the image supporting member comes in contact with the belt member, because of the difference between the circulating velocity of the belt member and the circumferential velocity of the image supporting member, the surface of the image supporting member which is slightly cut is always refreshed. Therefore, filming is prevented allowing the image quality to be maintained.
Preferably, the color image forming apparatus further comprises a cleaning member which abuts against a part of the belt member which is wound on the driven roller, to remove toner remained on the belt member.
In this configuration, the stretched condition of the intermediate transfer belt becomes stable even at the initial stage of image formation.
Preferably, the color image forming apparatus further comprises a secondary transfer position, formed on a part of the belt member which is wound on the driven roller, at which the toner images transferred from the image supporting members are secondarily transferred to a recording medium. The recording medium passes through the secondary transfer position upward from a lower part of the apparatus.
In this configuration, it is not necessary to provide the individual transfer roller inside of the transfer belt as shown in FIG. 5, so that it is possible to downsize the stretching structure of the transfer belt, thereby reducing the size of the apparatus.
Here, it is preferable that the color image forming apparatus further comprises a fixing section at which the secondarily transferred toner image is fixed on the recording medium. The fixing section is placed above the plurality of image supporting members.
In this configuration, heat or water vapor generated from the fixing section can be prevented from invading into the image forming section, so that it is possible to prevent occurrence of an image failure caused by temperature fluctuation, a registration shift caused by thermal expansion, an image failure caused by dew condensation, sticking of the contact parts, or the like.
In another embodiment according to the present invention, there is also provided a color image forming apparatus, comprising:
a transferring member;
at least one image supporting member, which supports a single color toner image thereon, and abutting onto the transferring member to define a transferring position at which the toner image is transferred onto either the transferring member or a recording medium held by the transferring member, while being rotated;
a first gear train, which rotates the transferring member;
a second gear train, which rotates the at least one image supporting member, while being connected to the first gear train with no branch; and
a single drive source, which drives the first gear train to thereby drive the second gear train,
wherein a circumferential velocity of the at least one image supporting member is higher than a circumferential velocity of the transferring member.
In this configuration, since the circumferential velocity of the transferring member is higher than the circumferential velocity of each image supporting member, the variation in the circumferential velocity difference between the respective image supporting members and the transferring member can be minimized. This makes it possible to stabilize the transferring condition of the toner image of each color from each of the image supporting members to the transferring member to provide a fine image.
Furthermore, since both of the first gear train and the second gear train are driven by the single drive source, the mechanical structure is remarkably simplified which allows for the apparatus size to be reduced.
Moreover, backlash in the first gear train does not occur even though the circumferential velocity of the transferring member is greater than the circumferential velocity of the respective image supporting members. Therefore, the above relationship between the rotation velocities can be reliably provided.
Particularly in the tandem-type color image forming apparatus, not only the driving mechanism for the plural image supporting members and the transferring member is remarkably simplified, but also a fine color image with no color shift can be reliably obtained.
In this embodiment, the surface hardness of the transferring member may also be greater than a surface hardness of the respective image supporting members. Alternatively, or in addition to this surface hardness relationship, a surface roughness of the belt member may be greater than a surface roughness of the respective image supporting members.
Furthermore, an abrasive may be applied on a surface of the transferring member.
In any of the above configurations, whenever the surface of the image supporting member comes in contact with the transferring member, because of the difference between the circumferential velocity of the transferring member and the circumferential velocity of the image supporting member, the surface of the image supporting member which is slightly cut is always refreshed. Therefore, the filming is prevented so that the image quality is maintained.